Conventional injectable formulations such as solution, suspension, and emulsion are quickly removed from the body after intramuscular or subcutaneous administration, and therefore frequent administration is essentially needed for treatment of chronic diseases. Microencapsulation has been developed to solve the problem, and referred to a production process for encapsulating drugs in microspheres (hereinafter, the term microsphere will include nanospheres) consisting of high molecular compounds. Microspheres are usually in a size of μm unit, and can be administered to a human or animal by intramuscular or subcutaneous injection. Further, microspheres can be produced to have a variety of drug release rates, so that the period of drug delivery can be controlled. Therefore, even if a therapeutic drug is administered only once, its effective concentration can be maintained over a long period of time, and the total administration amount of therapeutic drug can be minimized to improve the drug compliance in patients. Accordingly, world famous pharmaceutical companies are very interested in the production of polymeric microsphere loaded with drugs.
In the production of polymeric microspheres by microencapsulation, poly-d,l-lactide-co-glycolide (PLGA) has been most widely used as a high molecular compound. PLGA is a biocompatible high molecular compound that is hydrolyzed in vivo to be converted into nontoxic lactic acid and glycolic acid. Therefore, pharmaceutical industries have made extensive studies on the development of drug formulation using PLGA, and examples of current available microsphere product produced by using PLGA include Risperdal® Consta®, Sandostatin LAR®, Vivitrol®, and Lupron Depot®. Each of them is injected to a patient once to control the release of risperidone, octreotide acetate, naltrexone, and leuprolide acetate from 2 weeks to 4 months.
Such polymeric microspheres loaded with drugs can be conventionally produced by a solvent evaporation method or a solvent extraction method using an organic solvent such as methylene chloride and ethyl acetate.
First, the solvent evaporation method will be briefly described (see U.S. Pat. Nos. 6,471,996, 5,985,309, and 5,271,945). A drug is dispersed or dissolved in an organic solvent in which a high molecular compound is dissolved, and then emulsified in a dispersion medium such as water to produce an oil-in-water (O/W) emulsion. Then the organic solvent in the emulsion is diffused into a dispersion medium and evaporated across the air/water interface to form the polymeric microspheres loaded with drugs. At this time, in order to accelerate the diffusion of organic solvent into the dispersion medium, the organic solvent extraction method using reduced pressure, increased temperature, and an excessive amount of water is used. A dispersion organic solvent that is generally used to dissolve the high molecular PLGA is methylene chloride. Methylene chloride can dissolve well a PLGA copolymer with various molecular weights and lactide:glycolide ratios and can not mix well with water due to the low water solubility of 1.32% by weight. Thus, methylene chloride is a suitable solvent for the production of oil-in-water emulsion. Further, due to the low boiling point of 39.8° C., small amounts of methylene chloride molecules that diffused from emulsion liquid droplets into water are evaporated across the water/air interface. Such process is continuously repeated to remove methylene chloride from emulsion droplets, thereby forming microspheres. Finally, the residual methylene chloride present in microspheres is easily dried and removed due to its low boiling point. Based on the solvent evaporation method, a diagram showing the conversion of the emulsion droplets into microspheres is shown in FIG. 1. As shown in FIG. 1, a dispersion phase consisting of PLGA/drug/methylene chloride exists in the outer phase, such as water, as a form of oil-in-water emulsion (methylene chloride dissolved in water is represented as Δ) (A), and if the diffusion of methylene chloride in water and its evaporation are repeated, the emulsion droplets are converted into microparticles as shown in (B).
Likewise, even though methylene chloride is the most optimal solvent used for the production of emulsion in that it is very volatile, not mixed well with water, and has a lower boiling point than water, methylene chloride has the following problems: (a) it is a carcinogen proved by experiments; (b) it destroys the ozone layer in the atmosphere to generate a toxic environment, causing an increase in human skin cancer; (c) it is one of the 38 toxic and hazardous substances announced by the Agency for Toxic Substances and Disease Registry within the US Department of Health and Human Services; (d) a lot of time is required to completely remove methylene chloride in the emulsion droplets, since it has a low water solubility of about 1.32% by weight and only small amounts of methylene chloride are dissolved in water and evaporates. For example, in U.S. Pat. No. 6,884,435, the emulsion is stirred overnight to remove methylene chloride from the emulsion, and conditions such as increased temperature or reduced pressure in a reactor are introduced to shorten the production time of microspheres (see U.S. Pat. Nos. 3,691,090, 3,891,570, 6,270,700, and 6,572,894).
On the other hand, the solvent extraction method used to produce polymeric microspheres loaded with drugs is a method for effectively extracting the organic solvent in the emulsion droplets by using a large amount of solubilizing solvent. When the organic solvent is extracted from the emulsion droplets, the dissolved high molecular compounds are hardened to convert the emulsion droplets into microspheres. The solubilizing solvent that is generally used is water, and the degree of water solubility of the organic solvent greatly affects the amount of water needed. For example, methylene chloride has water solubility of 1.32% by weight, whereby a very large amount of water is needed for extracting methylene chloride in the emulsion. However, a large amount of wastewater containing methylene chloride is produced, in which the treatment of the wastewater becomes a problematic issue. Therefore, in the solvent extraction method, ethyl acetate, which has higher water solubility than methylene chloride, is mainly used. Since ethyl acetate has the water solubility of 8.7% by weight, it can be extracted by using a relatively small amount of water, as compared to methylene chloride, and it is advantageously a nonhalogenated organic solvent. However, its boiling point is 77° C. and much higher than 39.8° C., which is that of methylene chloride. Thus, ethyl acetate has a drawback that the residual solvent is hard to remove when dried. Furthermore, a PLGA polymer with a specific molecular weight and lactide:glycolide ratio has a characteristic of not dissolving easily in ethyl acetate.
Therefore, technologies simultaneously employing the solvent evaporation method and solvent extraction method are disclosed in U.S. Pat. Nos. 4,389,840, 4,530,840, 6,544,559, 6,368,632, and 6,572,894. That is, in the methods, the emulsion is produced, and then the organic solvent is partially removed by the evaporation process, and the residual organic solvent is removed by the solvent extraction method. For example, U.S. Pat. No. 4,389,840 discloses a method for producing microspheres, in which a drug and a PLGA polymer are dissolved in methylene chloride and then emulsified in water to produce oil in water-type emulsion, then 40 to 60% by weight of methylene chloride is removed by the evaporation process, and the residual methylene chloride is extracted using a large amount of water to produce microspheres.
However, since all of the organic solvents used in the known methods do not have sufficient high water solubility, excessively large amounts of water (over 10 times more than water solubility of organic solvent) should be used. Thus, a large-volume reactor is needed, and a large amount of wastewater containing organic solvent is produced, as a result, the cost for wastewater treatment is increased. Further, there is a problem that the residual organic solvent present in the microspheres is not effectively removed.
Accordingly, the present inventors have studied to solve the problems and a method for simply producing polymeric microspheres loaded with drugs. We have found that the polymeric microspheres loaded with drugs can be simply produced by dissolving a polymeric compound and a drug in a water-insoluble organic solvent to produce an emulsion, and converting into a water-soluble solvent through ammonolysis to harden emulsion droplets into microspheres, thereby completing the present invention.